This invention relates to palladium based dental alloys and, in particular, to alloys for use in porcelain-fused-to-metal restorations.
Porcelain-fused-to-metal restorations consist of a metallic sub-structure coated with a veneer of porcelain. Over the years various alloys have been proposed for the sub-structure of these restorations. Many of the early alloys used gold with some platinum or palladium as the main alloy ingredients. However, with the increases and fluctuations in the price of gold and platinum in recent years, other alloys have come to play major roles in this area. One series of alloys which has gained general acceptance is based on nickel, chromium and beryllium as the main ingredients. Another series of alloys, with which this invention is concerned, is based on palladium as the dominant element.
Alloys suitability for use in porcelain-fused-to-metal restorations must satisfy a plurality of demanding conditions. In particular, during torch melting and during the porcelain firing process, the alloy must form a suitable protective oxide. Also, the alloy should not be susceptible to "hot tearing" during the investment casting process.
Of primary importance, the alloy should exhibit good grain structure so as to give it high elongation, tensile strength and toughness. These properties are important in avoiding "hot tearing" and in providing a casting with good burnishability.
Recently, a number of palladium-based commercial alloys, sold under the trademarks CM Metal, Bond-On and Stroma, have become available. In the case of CM Metal and Bond-On, it is believed that the alloys are mixtures of palladium, indium, tin, cobalt, silicon, and, for CM Metal, perhaps some ruthenium. Stroma is believed to be a mixture of palladium, indium, tin and gallium. U.S. Pat. Nos. 4,261,744 and 4,319,877 are believed to relate to the CM Metal alloy. These patents describe alloys consisting of 75-85% by weight of Pd, 5-10% by weight of In, 5-10.5% by weight of Sn, up to 7.5% by weight of Co, Cr or Ni, up to 0.25% by weight of Si, and in the case of U.S. Pat. No. 4,319,877, 0.2-0.7% Ru. U.S. Pat. No. 4,319,877 states that the Ru is added to the alloy as a grain refiner and that the alloy is prepared in a vacuum furnace.
Surprisingly, in seeking to improve the grain structure of alloys similar to the above palladium-based commercial alloys, numerous difficulties were encountered. Specifically, as discussed in detail below, it was found that of the three grain refining elements--rhenium, ruthenium and iridium--only rhenium worked successfully, and then only when the alloy was prepared under vacuum or in an inert atmosphere.
Accordingly, it is one of the objects of this invention to provide a palladium-based dental alloy which is both grain refined and suitable for porcelain-fused-to-metal restorations. In particular, it is an object of this invention to provide a grain-refined palladium based dental alloy which has a low susceptibility to hot tearing. It is a further object of the invention to produce a palladium based dental alloy which forms a suitable oxide during torch melting and during the porcelain firing process.
The attainment of these and other objects of the invention is described below in connection with the description of the preferred embodiments of the invention.